1. Field of the Invention
The present invention relates to an optical pick-up apparatus. More particularly, the present invention relates to a diffraction element that improves tracking control, and an optical pick-up apparatus that uses the diffraction element.
2. Description of the Related Art
An optical pick-up apparatus is used in various devices such as compact disk players (CDs), digital versatile disk players (DVDs), CD-ROM drives, and similar device to read and write information from an optical recording medium (such as an optical disk) without contacting the medium. To write information, the optical pick-up irradiates the laser beam on the surface of the optical medium to form pits. To reproduce information, the optical pick-up optically reads the pit information on the optical medium and outputs it as electrical signals. To do this, the optical pick-up includes optical elements such as a laser diode which acts as a light source and generates a laser beam, a diffraction element, a beam splitter for controlling deviation of the laser beam, a plurality of lenses to form an optical path, a light detector for detecting a signal, and the like.
An optical pick-up apparatus focuses the beam spot on the disk surface by vertically controlling an objective lens, and controls the tracking of the beam on a track by horizontally controlling the objective lens. To control the focus and tracking, it is necessary to generate a focus error signal (hereinafter, referred to as a “FE signal”) and a tracking error signal (hereinafter, referred to as a “TE signal”). An astigmatism method is usually used to generate the FE signal. Various methods such as a push pull (hereinafter, referred to as a “PP”) method or a differential push pull (hereinafter, referred to as a “DPP”) method are used to generate the TE signal. The PP method is a method for detecting whether or not the beam spot is positioned at the center of a track by using the intensity detected in each region of a light detecting element that is divided into two regions. In the PP method, however, an offset is generated in the TE signal because the object lens is shifted radially during tracking.
To remedy this, the DPP method uses three beams to control tracking. In the DPP method, a main beam is irradiated at the center of the track, and two sub-beams are irradiated at the periphery of the groove at a predetermined distance away from the main beam. One beam is spaced apart in a radial direction and the other beam is spaced apart in a tangential direction of disk. The DPP method utilizes the difference among the 3 beams. Thus, it is possible to compensate for DC offset in the TE signal. When the DPP method is used for disks with different specifications for track pitch, however, the sub-beams cannot always be formed in the same groove as the main beam. That is, the DPP method is basically limited to a single track pitch, and cannot be used to compensate for DC offset of different types of disks.
Even so, in recent years, the DPP method which irradiates 3 beams on one track has been used. Japan Laid-open Patent Publication 2004-63073, which is hereby incorporated by reference in its entirety, discloses one example of such method, which will be described with reference to FIG. 1A and FIG. 1B.
A diffraction element 10 has three regions 12, 14 and 16, each having diffraction gratings at ¼ pitch (P/4) intervals with each other. Light incident on the diffraction element is diffracted into a main beam MB (the 0th order component) and two sub-beams SB (the ±1st order components). At this time, the main beam does not have any phase difference (0°), while the two sub-beams SB have phase differences of −90° and 90°, respectively. The three beams are aimed at one track T of the disk D, so that the TE signal can be detected from the three beams. However, in such a device, the respective regions 12, 14, and 16 have grooves that are offset with respect to each other, and a phase difference of ±90° occurs between the main beam MB and the sub-beams SB, thereby causing interference between the beams.
Due to the interference, the main beam MB forms a spot at the center of a track, but the leading and following sub-beams SB form spots that deviate from the center of track, and an image beam IB is formed at the spot of each sub-beam SB, as seen in FIG. 1B. As a result, the TE signal reflected from the disk D and detected by a light receiving element has errors, which lowers tracking precision.
Accordingly, there is a need for an improved diffraction element for an optical pick-up which improves tracking, and an optical pick-up device using the same.